feat(escalations): close out plan-locked wedge polish

Four items from the design-plan audit, all flagged as locked-design or
Codex corrections, shipped together so the GTM demo path covers them
end-to-end before bug bash.

1. Live AI assessment refresh on the magic-moment screen. Backend already
   publishes handoff_assessment_ready when enrich_escalation_async commits;
   wire the frontend listener so the senior sees the assessment populate
   without a manual reopen. New event type + onAssessmentReady handler on
   streamEscalations; AssistantChatPage opens a scoped SSE subscription
   whenever it tracks a handoff missing its assessment, refetches on match,
   and replaces magicHandoff / overlayHandoff in place. Closes the loop on
   the async-assessment commit e8ba74e.

2. Suggested-step chips below the chat input. Locked design from the plan
   (Codex correction). Chip strip renders above the composer post-claim
   when ai_assessment_data.suggested_steps[] is non-empty. Click prefills
   the input and focuses; first send or explicit X hides for the session.

3. Unread 6px dot on EscalationQueue cards. localStorage-persisted seen
   set (rf-escalation-seen, capped 200). Dot top-right when not seen.
   Cleared on open (card click) or claim (Pick Up) — NOT on hover, per
   Codex correction. Pick Up stops propagation so it doesn't double-fire.

4. Race-condition toast on claim conflict. The /claim endpoint previously
   silently overwrote claimed_by — both seniors thought they owned the
   session. New HandoffAlreadyClaimedError carries the winner's id/name/
   timestamp; claim_session rejects different-user re-claims (same-user is
   idempotent for double-click safety); endpoint returns 409 with
   structured detail. AssistantChatPage.handleStartHere extracts and
   surfaces "Already claimed by {name} {time_ago}." via toast, drops
   ?pickup=true, dismisses magic-moment so the loser flows back to queue.

Tests: 2 new unit tests in test_handoff_manager.py (conflict raises,
same-user idempotent). Full handoff + escalation suite (34 tests) green.
Frontend tsc -b clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/services/handoff_manager.py

@@ -36,6 +36,30 @@ from app.services.notification_service import notify
 logger = logging.getLogger(__name__)
 
 
+class HandoffAlreadyClaimedError(Exception):
+    """Raised when a senior tries to claim a handoff another senior already won.
+
+    Carries the winning claimer's id, display name, and claim timestamp so the
+    API layer can surface a "Already claimed by {name} {time_ago}" toast on
+    the losing client. The race story is the locked design — without this
+    exception the endpoint would silently overwrite `claimed_by` and both
+    seniors would think they own the session.
+    """
+
+    def __init__(
+        self,
+        claimed_by_id: UUID,
+        claimed_by_name: str,
+        claimed_at: datetime,
+    ) -> None:
+        super().__init__(
+            f"Handoff already claimed by {claimed_by_name} at {claimed_at.isoformat()}"
+        )
+        self.claimed_by_id = claimed_by_id
+        self.claimed_by_name = claimed_by_name
+        self.claimed_at = claimed_at
+
+
 class HandoffManager:
     """Unified park/escalate handoff management."""
 
@@ -398,14 +422,31 @@ class HandoffManager:
         handoff_id: UUID,
         claiming_user_id: UUID,
     ) -> SessionHandoff:
-        """Claim a handed-off session."""
+        """Claim a handed-off session.
+
+        If the handoff was already claimed by a *different* user (the race
+        story: two seniors clicking Pick Up simultaneously), raise
+        `HandoffAlreadyClaimedError` with the winning claimer's details so
+        the API can return 409 with the data the loser's toast needs. A
+        re-claim by the same user is idempotent.
+        """
         result = await self.db.execute(
-            select(SessionHandoff).where(SessionHandoff.id == handoff_id)
+            select(SessionHandoff)
+            .options(selectinload(SessionHandoff.claimed_by_user))
+            .where(SessionHandoff.id == handoff_id)
         )
         handoff = result.scalar_one_or_none()
         if not handoff:
             raise ValueError(f"Handoff {handoff_id} not found")
 
+        if handoff.claimed_by is not None and handoff.claimed_by != claiming_user_id:
+            claimer = handoff.claimed_by_user
+            raise HandoffAlreadyClaimedError(
+                claimed_by_id=handoff.claimed_by,
+                claimed_by_name=claimer.name if claimer else "another engineer",
+                claimed_at=handoff.claimed_at or datetime.now(timezone.utc),
+            )
+
         handoff.claimed_by = claiming_user_id
         handoff.claimed_at = datetime.now(timezone.utc)